Large interindividual variability in the in vitro formation of tamoxifen metabolites related to the development of genotoxicity

Br J Clin Pharmacol. 2004 Jan;57(1):105-11. doi: 10.1046/j.1365-2125.2003.01970.x.


Aims: To characterize the interindividual variability and the individual CYP involved in the formation of alpha-hydroxy-, N-desmethyl- and N-didesmethyl-tamoxifen from tamoxifen.

Methods: Microsomes from 50 human livers were used to characterize the interindividual variability in the alpha-hydroxylation, N-desmethylation and N-didesmethylation of tamoxifen. Selective inhibitors and recombinant enzymes were used to identify the forms of CYP catalysing these reactions.

Results: The rates of formation of alpha-hydroxy-, N-desmethyl- and N-didesmethyl-tamoxifen were highly variable, and correlated with each other (P < 0.0001). The respective ranges were 0.7-11.4, 25.7-411, and below the limit of quantification--4.4 pmol mg(-1) protein min(-1). Formation of all metabolites was observed with expressed recombinant CYP3A4, inhibited by troleandomycin (65, 77 and 35%, respectively, P < 0.05) and associated with CYP3A4 expression (rs = 0.612, rs = 0.585 and rs = 0.430, P < 0.01, respectively).

Conclusions: Formation of alpha-hydroxy-, N-desmethyl- and N-didesmethyl-tamoxifen in vitro is highly variable and mediated predominantly by CYP3A4.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Aged
  • Child
  • Child, Preschool
  • Cytochrome P-450 CYP3A
  • Cytochrome P-450 Enzyme System / genetics*
  • Cytochrome P-450 Enzyme System / metabolism
  • Female
  • Genetic Variation
  • Humans
  • Male
  • Microsomes, Liver / metabolism*
  • Middle Aged
  • Tamoxifen / analogs & derivatives*
  • Tamoxifen / antagonists & inhibitors
  • Tamoxifen / metabolism*


  • N,N-didesmethyltamoxifen
  • Tamoxifen
  • afimoxifene
  • Cytochrome P-450 Enzyme System
  • CYP3A protein, human
  • Cytochrome P-450 CYP3A
  • CYP3A4 protein, human
  • N-desmethyltamoxifen